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73NG FMS Speed/Speed tape difference

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73NG FMS Speed/Speed tape difference

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Old 4th May 2010, 11:53
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Join Date: Jun 2007
Location: Atlanta, GA USA
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73NG FMS Speed/Speed tape difference

Could someone please tell me why the FMS descent indicated speed will read 300, and when in the descent the number above the speed tape reads 299? Is there a way to fix it? Thanks for any ideas.
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Old 11th May 2010, 15:54
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Join Date: Dec 2009
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FMC SPD

Just open the spd window and set 300, now close the window the spd remains at 300. But the spd diff doesn't always occur dunno why though?

My guess is that the spd/M No set in the FMC and at the changeover altitude of M No/IAS if the max CAS is a Knot or two lower.
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Old 11th May 2010, 16:33
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Thanks for the fix fastcruise. I'll do some more research on max CAS and the switchover altitude idea. I doesn't happen all the time, but when it does happen I believe I have been in a VNAV PTH descent through the transition altitude. Good call.
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